18-09-2012, 10:14 AM
Comparison of Simulation and Experimental Results of UPFC used for Power Quality Improvement
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Abstract
This paper deals with digital simulation and
implementation of power system using UPFC to improve the
power quality. The UPFC is also capable of improving transient
stability in a power system. It is the most complex power
electronic system for controlling the power flow in an electrical
power system. The real and reactive powers can be easily
controlled in a power system with a UPFC. The circuit model is
developed for UPFC using rectifier and inverter circuits. The
control angle is varied to vary the real and reactive powers at
the receiving end. The Matlab simulation results are presented
to validate the model. The experimental results are compared
with the simulation results.
INTRODUCTION
The power-transfer capability of long transmission lines
are usually limited by large signals ability. Economic factors,
such as the high cost of long lines and revenue from the
delivery of additional power, give strong incentives to
explore all economically and technically feasible means of
raising the stability limit. On the other hand, the development
of effective ways to use transmission systems at their
maximum thermal capability has caught much research
attention in recent years. Fast progression in the field of
power electronics has already started to influence the power
industry. This is one direct outcome of the concept of flexible
ac transmission systems (FACTS) aspects, which has become
feasible due to the improvement realized in power-electronic
devices. In principle, the FACTS devices could provide fast
control of active and reactive power through a transmission
line. The unified power-flow controller (UPFC) is a member
of the FACTS family with very attractive features.
UPFC SYSTEM
A simplified scheme of a UPFC connected to an infinite
bus via a transmission line is shown in Fig.1. UPFC consists
of parallel and series branches, each one containing a
transformer, power-electric converter with turn-off capable
semiconductor devices and DC circuit. Inverter 2 is
connected in series with the transmission line by series
transformer. The real and reactive power in the transmission
line can be quickly regulated by changing the magnitude (Vb)
and phase angle (δb) of the injected voltage produced by
inverter 2. The basic function of inverter 1 is to supply the
real power demanded by inverter 2 through the common DC
link. Inverter 1 can also generate or absorb controllable
power [5],[6]. New method for improving transient stability
is given in [7].
SIMULATION RESULTS
Two bus system without compensation is shown in Fig 2a.
Sag is produced when an additional load is added. Voltage
across loads 1 and 2 are shown in Fig 2b. The real power and
reactive power waveforms are shown in Figures 2c and 2d
respectively.
Two bus system with UPFC is shown in Fig 3a. UPFC is
represented as a subsystem. The details of subsystem are
shown in Fig 3b. Voltage across loads 1 and 2 are shown in
Fig 3c. Real and reactive powers are shown in Figs 3d and 3e
respectively. UPFC using voltage and current sources are
shown in Fig4a. Converter 1 is represented as a shunt current
source and converter 2 is represented as a series voltage
source. Load voltage and current waveforms are shown in
Fig 4b. Real and reactive powers are shown in Fig 4c.
Variation of powers with the variation in the angle is given in
table 1. The real and reactive powers increase with the
increase in the angle of voltage injection.
EXPERIMENTAL RESULTS
Laboratory model of UPFC was designed and fabricated.
It is tested in the laboratory to obtain the experimental results.
Experimental set up is shown in Fig 5a. The hardware
consists of control circuit and power circuit. The control
board generates the pulses required by the MOSFETs. They
are generated by the 8 bit microcontroller. They are amplified
by using IR2110. The power circuit consists of rectifier and
inverter system. AC input voltage is shown in Fig 5b.
Rectifier output voltage is shown in Fig 5c. Driving pulses
are shown in Fig 5d. Load voltage after compensation is
shown in Fig 5e. From the figures 4b and 5e, it can be seen
that the simulation results coincide with the experimental
results.
CONCLUSION
In the simulation study, matlab simulink enviroment is
used to simulate the model of UPFC connected to a 3 phase
system. This paper presents the control & performance of the
UPFC used for power quality improvement. Voltage
compensation using UPFC is studied. The real and reactive
powers increase with the increase in angle of injection.
Simulation results show the effectiveness of UPFC to control
the real and reactive powers.